Since water absorbent resin particles lose fluidity as a result of moisture absorption, various investigations have been made, and a known representative method is a method of adding inorganic particles of clay, silica or the like. However, in this method, the added fine particles become dust and cause deterioration in the work environment, and the added fine particles cause a decrease in the water absorbent performance such as the water absorption capacity under load.
For that reason, a method of adding and wet mixing inorganic particles in the form of slurry, and a method of adding a coating agent such as a polymer have also be suggested; however, there have been limitations on the amount of addition and the like, because the water itself that is used for the addition causes a decrease in fluidity, or in order to avoid deterioration in the performance of the water absorbent resin particles by addition of the coating agent.
WO 00/10619 discloses a super absorbent polymer (SAP) which has a coagulation preventive function provided by addition of kaolin and has a reduced amount of dust. Specifically, disclosed is a super absorbent polymer obtained by adding 0.5% to 3% by mass of kaolin to a particulate SAP precursor material, followed by addition of a surface crosslinking aqueous solution containing ethylene carbonate thereto and heating of the mixture; by adding an aqueous solution containing kaolin in an amount of 2% to 3% by mass relative to the super absorbent polymer after a heating treatment; and by adding a surface crosslinking aqueous solution containing kaolin, followed by a heating treatment. The coagulation preventive function is dependent on the amount of addition of kaolin, and it is known that the coagulation preventive function is low at 0.5% by mass. Furthermore, in a qualitative dust evaluation, a tendency that the coagulation preventive function depends on the amount of addition of kaolin is observed. Also, in a method in which an aqueous solution containing kaolin is not added after surface crosslinking, the AUL is decreased.
Japanese Patent Application Laid-Open (JP-A) No. 62-223203 discloses a method for producing a highly swellable water absorbent polymer that is crosslinked using a crosslinking agent in the presence of a binary metal hydroxide having an anion exchange capability. Specifically, a highly swellable polymer which has a high water-absorbing speed, does not show stickiness after water absorption, and has gas permeability is obtained by adding hydrotalcite in an amount of about 5% to 20% by mass and a crosslinking agent to a water absorbent resin particle-containing solution obtained by reverse phase suspension polymerization, and performing azeotropic dehydration. According to the relevant patent document, it is described that co-presence of hydrotalcite at the time of the crosslinking reaction is essential, and simple mixing is not so effective. Regarding the reason for this, as a result of IR and X-ray analyses of the polymer thus obtained, it is speculated that a completely new composite has been formed, unlike the case of simple mixing.
WO 97/030109 discloses a method for controlling dust by physical coating with a hydrophobic organic anti-dusting agent; however, hydrophobic organic anti-dusting agents have problems such as a decrease in the water-absorbing speed because the surface of the water absorbing agent become hydrophobic.
Additives for imparting hygroscopic fluidity have problems such as generation of dust and deterioration in the water absorption performance such as a decrease in the water absorption capacity under load or the water-absorbing speed.
Therefore, there is a demand for an additive which suppresses blocking against moisture absorption without impairing the water absorption performance, suppresses the generation of dust, and has fewer restrictions on the production method.